Processes for the production of monofilaments from thermoplastic polymers without additional air quenching between the spinning head and spinning vat containing a liquid coolant are known in principle. Handbuch der Kunststoff-Extrusionstechnik II [manual of plastics extrusion II], Carl Hanser Verlag, Munich, Vienna, 1986, pages 295 to 319 describes the known process stages in detail. According to this reference, thermoplastic monofilaments (having a diameter of greater than 60 .mu.m) may be produced by spinning, for example in water, at a delivery speed of the finished monofilaments of at most 600 m/min.
Monofilaments of a substantially smaller cross-section and multi-filament fibres are directly spun into air as the coolant at a distinctly higher delivery speed using other processes. German published patent application DE 41 29 521 A1 thus describes an apparatus for high speed spinning multi-filament fibres at a windup speed of at least 2000 m/min.
In the latter-stated process. monofilaments or multi-filament fibres are spun into air and directly wound up. One particular feature of this process is the cooling apparatus used therein. It consists of a porous pipe open in the direction of spinning and arranged concentrically relative to the tow. Given the elevated windup speed, no cooling medium is actively supplied. The process described therein relates to filament yarns with the filaments having an individual linear density of 0.1 to 6 dtex and is not applicable to monofilaments having a diameter of greater than 50 .mu.m (approx. 22 dtex).
International patent application WO 91/11547 describes a process and apparatus for high speed spinning of monofilaments having an individual linear density of 1 to 30 dtex (corresponding to approx. 10 to 57 .mu.m). In this process, the melt-spun monofilaments are cooled with an air blast, drawn over a friction element, provided with a finish and wound up at a speed of up to 6000 m/min. This process differs fundamentally from the process according to DE 41 29 521 A1 with regard to the active cooling of the monofilament by an air blast and by the use of a friction element, by means of which fibre tension is adjusted.
Both direct spinning/stretching processes (according to DE 41 29 520 A1 and WO 91/11547) are in principle limited to the production of small diameter monofilaments (i.e. having a fibre diameter of &lt;57 .mu.m) due to unfavourable heat dissipation brought about by air cooling and the poor internal thermal conductivity of the fibres.
German patent application bearing the file number P 43 36 097.1 describes a continuous high speed production process for the production of melt-spun monofilament fibres having a diameter of 60 .mu.m to 500 .mu.m. In this process, the polymer fibres formed are laterally quenched over a zone of 1 to 10 cm beneath the spinning head with temperature-controlled air from nozzles in order to stabilise the smooth running of the fibres. After the air cooling, the polymer filaments are cooled in a liquid bath.
The surface of the melt fibres which have only passed through a short air zone, for example as in the last-stated process, and have then been directly spun into a liquid exhibits a texture similar to orange peel. The monofilaments exhibit a loss of strength and a wide dispersion of their knot strength.
Moreover, the sudden cooling of the monofilaments in the cooling liquid gives rise to a pronounced core/shell structure in the filaments which also degrades the mechanical properties of the filaments.
Due to the unfavourable dissipation of heat on air cooling and poor internal thermal conductivity in such processes in which only air is used as the cooling medium, monofilament production is limited to a diameter of &lt;57 .mu.m.
Additional air quenching between the spinning head and spinning bath by nozzles over an air zone of 1 to 10 cm (corresponding to DE 43 36 097) gives rise to satisfactory textile characteristics in thin monofilaments (having a diameter of &lt;200 .mu.m) spun at high speed. Use of the stated air cooling zone is not sufficient for thicker monofilaments. Moreover, the process is extremely sensitive to air movement in the fibre forming zone, so impairing the operational reliability of the plant.